Vol. 60.] ROCKS OF THE BORROWDALE VOLCANIC SERIES. 



89 



Borrowdale graphite-mine, and resembles in texture an intrusive 

 rock in Sourmilk-Gill Combe. It is not a garnet-bearing rock. It 

 probably extends some distance eastward, for a very similar rock is 

 met with at the same horizon on Watendlath Fell. 



Garnet-beariug ashes and lavas are extensively developed on the 

 high ground between Borrowdale and Thirlmere, but very little 

 attention has been given to them. The garnet-lavas on the Dock- 

 Tarn plateau greatly resemble the garnet-lava Xo. 10 of the Falcon- 

 Crag Series (p. 87) : and it is quite possible that in them we have 

 the uppermost members of that series. 



The next great group of the Eycott Lavas and associated 

 ashes occurs immediately to the south of them, and forms prominent 

 escarpments on both sides of the valley. Garnets do not occur in 

 these lavas, except in the neighbourhood of intrusive masses such 

 as the Carrock-Fell rocks, where they are doubtless the product of 

 contact-metamorphism. 



VII. The • Streaky ' Rocks of the Central Mountain-District. 



We have next to deal with the group of ' streaky '" rocks. A good 

 idea of the mutual relations of these rocks may be gathered from 

 the appended diagrammatic section through Rosthwaite Fell. 



x. 



Fig. 2. — Diagrammatic section through Rosthwaite Fell. 



Stonethwaite 

 Church 



: Wood 



(//) = garnet-breccia. &=rasli and breccia. 



gl= garnet-lava. e — Eycott Lavas. 



s= intrusive sill. st = ' streaky ' rocks. 



gp=garnet-quartz-porphyry. 



The term ' streaky ' has been applied to these rocks from the 

 characteristic appearance of the weathered surfaces. These show 

 either a series of parallel wavy lines, or a multitude of lenticular 

 inclusions with the same orientation. Strictly speaking, rocks 

 showing this ' streaky ' character occur in all parts of the district 

 and at all geological horizons, but a well-developed band of them 

 is generally found above the Eycott Lavas. 



This type of structure might arise in various ways. 



(1) By infiltration of substances along definite planes. This is 

 frequently noticed, not only in the typical ' streaky ' rocks, but in 

 rocks of all geological horizons. The infiltrated substances belong 

 to the following : — quartz, calcite, chlorite, epidote, and ilmenite. 

 These are often introduced along the bedding-planes, and the 

 parallel streaky lines may be separated by considerable intervals. 

 Good examples are afforded by certain lava-like rocks on Gosforth 



